Reflectometers have been constructed featuring optical arrangements of lenses, filters, apertures, a radiation source, and detector. Examples are described in U.S. Pat. Nos. 4,219,529; issued Aug. 26, 1980 and 4,224,032, FIGS. 9 and 10; issued on Sept. 23, 1980. In such arrangements, the separate components, such as the lenses, have to be accurately located and mounted to insure proper light ray alignment and focusing.
Although such reflectometers have been successfully used, there has been a need for a simpler arrangement in which the number of components is reduced and the positioning of the components simplified. Particularly, such a need exists in the field of portable instruments, such as those used by individuals, either at home or while traveling. For example, in the case of a reflectometer used as a portable analyzer, there is a need for a reflectometer that is thin enough to fit in the user's pocket.
U.S. Pat. No. 3,536,927, issued on Oct. 27, 1970, describes a simplified reflectometer, wherein a light source and a number of detectors are mounted within a light guide. The light guide acts to direct the radiation to a plurality of emitting areas, and radiation reflected by the test object is detected.
Several disadvantages exist in devices such as are shown in the aforesaid patent. One disadvantage is that no provision is made to exclude the detection of specular reflectance. Instead, light is randomly delivered within the light guide at all angles from the light source, producing radiation that illuminates the test element at a number of angles. Because the emitted light occurs at such a variety of angles, encouraged by multiple reflections within the light guide, no provision can be made to effectively shield the detector means from specular reflectance. Specular reflection is a significant problem with test elements that have a transparent exterior surface, such as a support, that is scanned by the reflectometer. Examples of elements having such a construction appear in U.S. Pat. No. 3,992,158, issued Nov. 16, 1976. Such transparent exterior surfaces specularly reflect about 4% of the incident radiation, regardless of the absorption of light that occurs within the test element. Such specular reflection represents a significant noise factor that must be eliminated in order for highly accurate readings to be made of low-level analytes.